SSR doesn't only provide identity & altitude information. I also gives range and bearing. I have never seen a radar display that corrects for slant angle, but that's not to say that they aren't in existence.
One interesting aspect to consider is when the secondary data is being combined with primary data where the 2 radars are sited in different locations. In this case there can be a significant different difference in the computed position of the 2 radar returns.